Patent application title: PHOTOMASK INSPECTION APPARATUS

Abstract:

According to one aspect of the present invention, there is provided a
photomask inspection apparatus which observes a pattern provided on a
mask substrate of a mask to inspect the mask including an object lens,
and a liquid that is present between a last lens in the side closer to
the mask of the object lens and the mask.

Claims:

1. A photomask inspection apparatus which observes a pattern provided on a
mask substrate of a mask to inspect the mask, comprising:an object lens;
anda liquid that is present between an last lens in the side closer to
the mask of the object lens and the mask.

2. The photomask inspection apparatus according to claim 1, wherein the
object lens is placed on the opposite side of the mask substrate from a
pattern surface thereof.

3. The photomask inspection apparatus according to claim 2, wherein the
object lens is placed above the mask and is provided with a supply unit
to supply the liquid to a non-pattern surface of the mask substrate
opposite to the pattern surface and a sucking unit to suck the liquid.

4. The photomask inspection apparatus according to claim 2, wherein the
object lens is placed below the mask.

5. The photomask inspection apparatus according to claim 4, wherein the
object lens has an outlet to supply the liquid, and the outlet is
provided upstream in a scan direction.

6. The photomask inspection apparatus according to claim 5, wherein a
plurality of the outlets are provided at least upstream and downstream in
the scan direction, and the object lens has a notch to drain the liquid
in a direction perpendicular to the scan direction.

7. The photomask inspection apparatus according to claim 5, wherein the
object lens has an inlet surrounding the outlet to suck in the liquid.

8. The photomask inspection apparatus according to claim 7, wherein the
object lens has a gas outlet surrounding the inlet to discharge gas.

9. The photomask inspection apparatus according to claim 1, further
comprising:a container to contain the liquid,wherein the mask is placed
in the container so that a surface of the mask opposite to a pattern
surface thereof is in contact with the liquid.

10. The photomask inspection apparatus according to claim 9, wherein the
pattern surface facing upwards, the object lens is placed below the
container, the inspection apparatus further comprising:a measuring unit
for measuring the distance to the pattern surface of the mask in the
container.

11. The photomask inspection apparatus according to claim 1, wherein the
mask is a mask with a pellicle.

12. The photomask inspection apparatus according to claim 1, wherein the
mask is an imprint mask.

13. The photomask inspection apparatus according to claim 4, further
comprising:a support to hold the mask,wherein a step corresponding in
outline to the support is formed in the top of a body of the object lens.

14. The photomask inspection apparatus according to claim 13, wherein two
of the steps are formed respectively in opposite sides of the top of the
body of the object lens.

15. The photomask inspection apparatus according to claim 13, wherein the
step is formed throughout the periphery of the top of the body of the
object lens.

16. The photomask inspection apparatus according to claim 13, wherein a
mask scan direction during inspection of the mask is parallel to a
longitudinal direction of a maximum pattern area of the mask.

17. The photomask inspection apparatus according to claim 1, wherein the
object lens is rotatably held so that when the mask and the object lens
moves relatively to each other, the orientation of the object lens around
its optical axis can be changed according to the direction of the
movement.

18. The photomask inspection apparatus according to claim 3, further
comprising a plane plate positioned adjacent to at least two of four
sides of the mask and substantially level with the non-pattern surface of
the mask substrate of the mask.

19. The photomask inspection apparatus according to claim 1, wherein the
object lens has a plurality of outlets provided surrounding the last
lens, and when the object lens is stationary relative to the mask, the
liquid is supplied from the plurality of outlets.

20. The photomask inspection apparatus according to claim 1, wherein
refractive index of the liquid is substantially equal to that of the mask
substrate.

Description:

BACKGROUND OF THE INVENTION

[0001]1. Field of the Invention

[0002]The present invention relates to a photomask inspection apparatus
for inspecting photomasks (or also called reticles but herein simply
called masks) used in a semiconductor manufacturing process.

[0003]2. Description of Related Art

[0004]Generally, there are widely known two mask defect inspecting method:
an inspection method which compares a mask pattern and design data
(generally called a die-to-database comparing method) and an inspection
method which compares two mask patterns (generally called a die-to-die
comparing method). In either of these inspection methods, an image of a
mask pattern is detected with a microscope. At this time, if an optical
microscope is used, the mask pattern needs to be illuminated with light.
For the light source (i.e., a mask inspection light source), there are
two main categories: the use of a lamp and the use of a laser. In
photomask inspection apparatuses using a laser, a continuous laser that
generates continuous laser light is usually used.

[0005]At present, photomask inspection apparatuses using continuous laser
light of 257 nm in wavelength (the second harmonic of the wavelength of
514 nm that is the maximum output line of an argon laser) as the
inspection light source laser are available in the market, which are
described in, e.g., Proceedings of SPIE, vol. 5446, pp. 265-278, 2004 or
Toshiba Review, vol. 58, No. 7, pp. 58-61, 2003.

[0006]As feature sizes of semiconductor devices become finer, patterns on
the masks become finer. Accordingly, for improved sensitivity in
detecting defects, there is a demand for use of a shorter wavelength for
the light sources of photomask inspection apparatuses as well. The mask
inspection light source of the next generation is required to be a light
source having a wavelength of 200 nm or less. For example, a photomask
inspection apparatus has been developed which has generated therein
ultraviolet laser light having a wavelength of 198.5 nm that is the
summation frequency of the frequencies of the second harmonic of an argon
laser of 488 nm in wavelength and a fiber laser of 1064 nm in wavelength
to use as the mask inspection light. Such a photomask inspection
apparatus is disclosed in, e.g., Japanese Unexamined Patent Application
Publication No. 2006-73970 or Proceedings of SPIE vol. 5992, pp.
43-1-43-8, 2005.

[0007]In the structure of typical KrF or ArF lithography masks used in the
semiconductor manufacturing process, one face of a mask substrate 801
made of synthetic quartz is a pattern surface 802 like a mask 810 shown
in FIG. 13. Spacer 803a, 803b are provided on the periphery of the mask
substrate 801. A pellicle 804, a transparent thin film, is applied onto
the spacer 803a, 803b so that the pattern surface 802 is kept in a sealed
space, thereby preventing dust outside the mask 810 from sticking to the
pattern surface 802. Because the pellicle 804 is formed of an extremely
thin polymer of about 1 μm in thickness, there is the problem that the
pellicle is easy to be torn.

[0008]FIG. 14 shows the usual configuration of a conventional photomask
inspection apparatus. As shown in FIG. 14, the conventional photomask
inspection apparatus 800 observes an observed area in the pattern surface
802 with an object lens 807 placed directly above the pellicle 804 of the
mask 810. Laser light L81 as illuminating light is reflected downward by
a polarization beam splitter 805, passes through a quarter wavelength
plate 806 to be converted to circular polarization, passes through the
object lens 807, and is irradiated onto the observed area in the pattern
surface 802. The illuminated observed area in the pattern surface 802 is
enlarged by the object lens 807 and a projection lens 808 and projected
onto a two-dimensional sensor 809. The way that illuminating light is
made incident from the object lens 807 side is called reflected
illumination. Meanwhile, the way that the pattern surface 802 is
illuminated from the opposite side thereof from the object lens 807 is
called transmitted illumination.

[0009]In the lithography technology of the semiconductor manufacturing
technology, an ArF excimer laser of 193.4 nm in wavelength is widely used
as the light source for exposure. An exposure technique using this is
called ArF lithography. As lithography technology for realizing even
finer feature sizes, an exposure technique called liquid immersion where
the gap between the projection lens of an exposure apparatus and a wafer
is filled with water is becoming widely used. This is also called ArF
liquid immersion exposure, ArF liquid immersion, or the like. FIG. 15
shows the usual configuration of a conventional liquid immersion exposure
apparatus. As shown in FIG. 15, in a liquid immersion exposure apparatus
900, the gap between a lens (not shown) at the lower end of a reduction
projection optical system 903 used to project a reduced image of a mask
901 onto a wafer 902 and the wafer 902 is filled with pure water 904.

[0010]The wafer 902 is mounted on a wafer stage 905, and with the gap
between the wafer 902 and the reduction projection optical system 903
being filled with pure water 904, the wafer 902 moves back and forth. The
pure water 904 is supplied from a pure water supply unit 906 and sucked
into a pure water sucking unit 907 so as to usually fill the space under
the reduction projection optical system 903. The ArF liquid immersion
exposure is described in, e.g., Electric Journal, pp. 73-74, May 2004,
and an inspection apparatus for wafers or the like using a liquid
immersion optical system is described in, e.g., Japanese Unexamined
Patent Application Publication No. 2005-83800, No. 2005-338027, and No.
2006-171186.

[0011]In order to improve the resolving power, i.e., sensitivity of a
photomask inspection apparatus, it is inevitable to use a light source of
a shorter wavelength. In the future, even a laser of the above-mentioned
wavelength of 198.5 nm will not suffice in sensitivity. Even if an ArF
excimer laser of 193.4 nm in wavelength or the like can be used for the
mask inspection light source, they will not be enough in sensitivity to
inspect a 32-nm generation of masks.

[0012]Accordingly, considering the application of the liquid immersion
technique to photomask inspection apparatuses as with the exposure
technology, the problem below exists, and hence it has been extremely
difficult to apply the liquid immersion technique. The reason is that, as
seen from the conventional photomask inspection apparatus 800 of FIG. 14,
if an attempt is made to apply the liquid immersion technique to
photomask inspection, the space between the pattern surface 802 and the
object lens 807 will be filled with water, and hence the pellicle 804
cannot be used. Of course, at a stage before applying the pellicle to
them in the production process of masks, such liquid immersion technique
can be used, but in the photomask inspection where masks finished by
applying the pellicle thereto are inspected, the liquid immersion
technique cannot be used. There is the problem that in the conventional
photomask inspection apparatus, it is difficult to improve the resolution
and sensitivity.

[0013]An object of the present invention is to provide a photomask
inspection apparatus of high resolution.

SUMMARY OF THE INVENTION

[0014]According to a first aspect of the present invention, there is
provided a photomask inspection apparatus which observes a pattern
provided on a mask substrate of a mask to inspect the mask including an
object lens, and a liquid that is present between a last lens in the side
closer to the mask of the object lens and the mask. By this means,
resolution can be improved.

[0015]According to a second aspect of the present invention, the object
lens is placed on the opposite side of the mask substrate from a pattern
surface thereof in the photomask inspection apparatus. By this means, the
liquid can be easily removed from the mask.

[0016]According to a third aspect of the present invention, the object
lens is placed above the mask and is provided with a supply unit to
supply the liquid to a non-pattern surface of the mask substrate opposite
to the pattern surface and a sucking unit to suck the liquid in the
photomask inspection apparatus. By this means, the configuration of the
apparatus becomes simple.

[0017]According to a fourth aspect of the present invention, the object
lens is placed below the mask in the photomask inspection apparatus. By
this means, the configuration of the apparatus becomes simple.

[0018]According to a fifth aspect of the present invention, the object
lens has an outlet to supply the liquid, and the outlet is provided
upstream in a scan direction in the photomask inspection apparatus. By
this means, the configuration of the apparatus becomes simple.

[0019]According to a sixth aspect of the present invention, a plurality of
the outlets are provided at least upstream and downstream in the scan
direction, and the object lens has a notch to drain the liquid in a
direction perpendicular to the scan direction in the photomask inspection
apparatus. By this means, the configuration of the apparatus becomes
simple.

[0020]According to a seventh aspect of the present invention, the object
lens has an inlet surrounding the outlet to suck in the liquid in the
photomask inspection apparatus. By this means, the configuration of the
apparatus becomes simple.

[0021]According to an eighth aspect of the present invention, the object
lens has a gas outlet surrounding the inlet to discharge gas in the
photomask inspection apparatus. By this means, the configuration of the
apparatus becomes simple.

[0022]According to a ninth aspect of the present invention, there is
provided the photomask inspection apparatus further including a container
to contain the liquid, wherein the mask is placed in the container so
that a surface of the mask opposite to a pattern surface thereof is in
contact with the liquid. By this means, the masks can be observed stably.

[0023]According to a tenth aspect of the present invention, there is
provided the photomask inspection apparatus further including a measuring
unit for measuring the distance to the pattern surface of the mask in the
container, the pattern surface facing upwards, and the object lens is
placed below the container. By this means, even if variation in thickness
between masks exists, the masks can be observed stably.

[0024]According to an eleventh aspect of the present invention, the mask
is a mask with a pellicle in the photomask inspection apparatus.
Therefore, the mask with the pellicle can be observed with high
resolution.

[0025]According to a twelfth aspect of the present invention, the mask is
an imprint mask in the photomask inspection apparatus. Therefore, the
imprint mask can be observed with high resolution.

[0026]According to a thirteenth aspect of the present invention, there is
provided the photomask inspection apparatus further including a support
to hold the mask, wherein a step corresponding in outline to the support
is formed in the top of a body of the object lens. By this means, when
the object lens is located at an edge of a mask, a claw of an arm holding
the mask from below can be received in the step.

[0027]According to a fourteenth aspect of the present invention, two of
the steps are formed respectively in opposite sides of the top of the
body of the object lens in the photomask inspection apparatus. By this
means, when the object lens is located at either of opposite edge sides
of a mask, a claw of an arm holding the mask from below can be received
in the step.

[0028]According to a fifteenth aspect of the present invention, the step
is formed throughout the periphery of the top of the body of the object
lens in the photomask inspection apparatus. By this means, when the
object lens is located at any of four edge sides of a mask, a claw of an
arm holding the mask from below can be received in the step.

[0029]According to a sixteenth aspect of the present invention, a mask
scan direction during inspection of the mask is parallel to a
longitudinal direction of a maximum pattern area of the mask in the
photomask inspection apparatus. By this means, distances between edges of
the pattern area and edge sides of the mask are long, and hence the
object lens does not interfere with the support holding the mask.

[0030]According to a seventeenth aspect of the present invention, the
object lens is rotatably held so that when the mask and the object lens
moves relatively to each other, the orientation of the object lens around
its optical axis can be changed according to the direction of the
movement. By this means, even if there is only one outlet, the outlet can
always be placed upstream in a scan direction, and hence the structure
for the supply and suction of pure water for the object lens can be
simplified.

[0031]According to an eighteenth aspect of the present invention, there is
provided the photomask inspection apparatus further including a plane
plate positioned adjacent to at least two of four sides of the mask and
substantially level with the non-pattern surface of the mask substrate of
the mask. By this means, if the liquid flows to an edge of the mask,
although it spreads over the plane plate, the liquid can be prevented
from flowing around to the pellicle side of the mask.

[0032]According to a nineteenth aspect of the present invention, the
object lens has a plurality of outlets provided surrounding the last
lens, and when the object lens is stationary relative to the mask, the
liquid is supplied from the plurality of outlets. By this means, also in
review or the like where a mask is stopped, the entire surface of the
last lens is covered by the liquid.

[0033]According to a twentieth aspect of the present invention, wherein
refractive index of the liquid is substantially equal to that of the mask
substrate in the inspection apparatus.

[0034]According to the present invention, a photomask inspection apparatus
of high resolution can be provided.

[0035]The above and other objects, features and advantages of the present
invention will become more fully understood from the detailed description
given hereinbelow and the accompanying drawings which are given by way of
illustration only, and thus are not to be considered as limiting the
present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0036]FIG. 1 shows the configuration of a photomask inspection apparatus
according to embodiment 1;

[0037]FIG. 2 shows the configuration of a photomask inspection apparatus
according to embodiment 2;

[0038]FIG. 3 shows the configuration of part of a photomask inspection
apparatus according to embodiment 3;

[0039]FIG. 4A shows the configuration of another object lens usable in the
present invention;

[0040]FIG. 4B shows a preferable example of scan using the object lens of
FIG. 4A.

[0041]FIGS. 5A and 5B show the configuration of yet another object lens
usable in the present invention;

[0042]FIGS. 6A and 6B show the configuration of still another object lens
usable in the present invention;

[0043]FIGS. 7A, 7B and 7C illustrate the thickness of pure water in the
photomask inspection apparatus according to the present invention;

[0062]FIGS. 25A and 25B show the configuration of an object lens used in
the present invention; and

[0063]FIGS. 26A, 26B and 26C illustrate pure water supply operation at
review with the object lens of FIG. 25A.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0064]Embodiments of the present invention will be described below with
reference to the drawings. The description below will be made to present
preferred embodiments of the present invention, and the invention is not
intended to be limited in scope to the embodiments below. The same
reference numerals indicate substantially the same parts.

Embodiment 1

[0065]A photomask inspection apparatus according to embodiment 1 of the
present invention will be described with reference to FIG. 1. FIG. 1
shows the configuration of a photomask inspection apparatus 100 according
to the present embodiment. As shown in FIG. 1, the photomask inspection
apparatus 100 comprises an inspection light source 130, a half mirror
101, lenses 102a to 102d, homogenizing optical systems 103a, 103b, a
polarization beam splitter 104, a quarter wavelength plate 105, a
condenser lens 106, a projection lens 107, a two-dimensional photosensor
109, mirrors 111a, 111b, an object lens 112, a water bath 113, and pure
water 114. The photomask inspection apparatus 100 according to the
present embodiment is for inspecting a finished mask 108 comprising a
frame 108d provided so as to surround a pattern surface 108c formed on a
mask substrate 108a and a pellicle 108b applied to this. In this
embodiment, synthetic quartz of about 1.5608 in refractive index is used
as the mask substrate 108a.

[0066]The photomask inspection apparatus 100 uses laser light L01 of 193
nm in wavelength from the inspection light source 130 for illumination.
The laser light L01 is incident on the half mirror 101 and divided into
lights in two directions. One of the divided laser lights is used as
transmitted illumination light and the other is used as reflected
illumination light.

[0067]First, the transmitted illumination will be described. Laser light
L02 that has passed through the half mirror 101 travels through the lens
102a with converging and is incident on the homogenizing optical system
103a. The homogenizing optical system 103a is, for example, a fiber
bundle that is a bundle of fibers. Laser light L03 homogenized in spatial
light intensity distribution is output by the homogenizing optical system
103a and passes through the lens 102b to be collimated and is deflected
by the mirror 111b to be incident on the condenser lens 106, which
condenses it into laser light L04. The laser light L04 illuminates an
observed area in the pattern surface 108c of the mask 108. The condenser
lens 106 is placed directly above the pellicle 108b of the mask 108.

[0068]Next, the reflected illumination will be described. Laser light L05
that has been reflected by the half mirror 101 is reflected by the mirror
111a and passes through the lens 102c to be incident on the homogenizing
optical system 103b. Laser light L06 homogenized in spatial light
intensity distribution is output by the homogenizing optical system 103b,
passes through the lens 102d to be collimated, and is incident on the
polarization beam splitter 104. Because it is S waves, laser light L07 is
reflected by the polarization beam splitter 104 and travels upward as
indicated by laser light L08. The laser light L08 passes through the
quarter wavelength plate 105 to be circularly polarized into laser light
L09. The laser light L09 is incident on the object lens 112 fixed in the
middle of the water bath 113 and passes through the pure water 114 and
the mask substrate 108a of the mask 108 and illuminates the pattern
surface 108c from below. The water bath 113 is a container containing the
pure water 114.

[0069]An optical system for observing an observed area in the pattern
surface 108c of the mask 108 will be described below. Laser light L09
imparted with an optical image originating from an observed area in the
pattern surface 108c illuminated by the transmitted illumination or the
reflected illumination is output downward from the object lens 112, which
laser light contains optical information in the form of a spatial light
intensity distribution. This laser light L09 comprises reflected light
reflected by the mask 108 and transmitted light transmitted by the mask
108. The laser light L09 reflected by the mask 108 is circularly
polarized in a direction opposite to that of the laser light L09 incident
on the mask 108 and hence, passing through the quarter wavelength plate
105 again, becomes here P waves. Thus, the reflected light from the
pattern surface 108c of the mask 108 passes through the polarization beam
splitter 104. As a result, the reflected light passes through the
projection lens 107 and strikes the two-dimensional photosensor 109. That
is, an image of the observed area in the pattern surface 108c is enlarged
by the ratio of the focal distance of the projection lens 107 to the
focal distance of the object lens 112 and enlargement projected onto the
two-dimensional photosensor 109. The laser light L09 that has passed
through the mask 108 is also output from the object lens 112 and passes
through the same optical path to be incident on the two-dimensional
photosensor 109.

[0070]In the photomask inspection apparatus 100 according to the present
embodiment, the water bath 113 contains the pure water 114. The mask 108
is placed in the water bath 113 such that the surface of the mask
substrate 108a opposite to the pattern surface 108c contacts the pure
water 114 contained therein. The object lens 112 is placed on the
opposite side of the mask 108 from the pattern surface 108c. That is, the
object lens 112 is placed under the surface of the mask substrate 108a
opposite to the pattern surface 108c, and the space between the object
lens 112 and the mask substrate 108a is filled with the pure water 114.
The object lens 112 includes multiple lenses. The space between a last
lens on the mask 108 side included in the object lens 112 and the mask
substrate 108a is filled with the pure water 114. Thus, an air layer is
not formed between the pattern surface 108c and the object lens 112. By
this means, only the pure water 114 of about 1.44 in refractive index and
the mask substrate 108a of about 1.56 in refractive index are sandwiched
between the pattern surface 108c and the object lens 112. As such,
according to the present invention, the liquid immersion technique can be
applied to photomask inspection as well.

[0071]In the present embodiment, because the pattern surface 108c observed
is on a quartz substrate, the sensitivity can be raised to as high a
level as when inspected with a light source having a wavelength equal to
the wavelength of that inspection light source divided by the refractive
index of synthetic quartz. For example, for a light source wavelength of
193 nm, because the refractive index of synthetic quartz is about 1.5608,
sensitivity of the same level as when inspected with a light source of a
minimum of about 124 nm in wavelength is obtained. In reality, depending
on the refractive index of liquid in use, with water having a refractive
index of 1.436 at a wavelength of 193 nm, sensitivity is of the same
level as when inspected with a light source of 134 nm in wavelength. By
this means, the resolution can be increased by a factor of about 1.56,
which equals the index of the synthetic quartz in contact with the
pattern surface 108c, as compared with the conventional apparatus using
an inspection light source of the same wavelength.

[0072]If a xenon mercury lamp of 248 nm is used, because the refractive
index of synthetic quartz is 1.5086 at a wavelength of 248 nm, resolution
of substantially the same level as when using a light source of 164 nm in
wavelength will be obtained by applying the present invention.

[0073]Because during inspection the mask 108 can freely move along a plane
in the water bath 113, the center and around it of the object lens 112
can be positioned opposite every part of the pattern surface 108c. By
this means, the almost entire mask 108 can be observed. Further, the
water bath 113 may be sealed to the object lens 112, thereby preventing
the leakage of the pure water 114. The sealing material can be an elastic
substance such as rubber.

[0074]In the present invention, where the object lens 112 is placed below
the mask 108, if the object lens 112 is large, it may be difficult to
support the mask substrate 108a from below, in which case the mask
substrate 108a may be held by vacuum suction from above at the outside of
the pellicle 108b.

[0075]In the present embodiment, the object lens 112 has its focal
distance of about 3 mm. Meanwhile, the focal distance of the projection
lens 107 is about 300 mm. As a result, an image of part of the pattern
surface 108c is enlarged by a factor of about 100 and projected onto the
two-dimensional photosensor 109. A CCD, a TDI, or the like is suitable as
the two-dimensional photosensor 109.

Embodiment 2

[0076]A photomask inspection apparatus according to embodiment 2 of the
present invention will be described with reference to FIG. 2. FIG. 2
shows the configuration of a photomask inspection apparatus 200 according
to the present embodiment. As shown in FIG. 2, the photomask inspection
apparatus 200 comprises an inspection light source 230, a half mirror
201, lenses 202a to 202d, homogenizing optical systems 203a, 203b, a
polarization beam splitter 204, a quarter wavelength plate 205, a
condenser lens 206, a projection lens 207, a two-dimensional photosensor
209, mirrors 211a, 211b, an object lens 212, pure water 214, a pure water
supply unit 215, and a pure water sucking unit 216. Duplicate description
with the embodiment 1 will be omitted.

[0077]The photomask inspection apparatus 200 according to the present
embodiment is also for inspecting a finished mask comprising a frame 208d
provided so as to surround a pattern surface 208c formed on a mask
substrate 208a and a pellicle 208b applied to this. In this embodiment,
synthetic quartz of about 1.5608 in refractive index is used as the mask
substrate 208a. The mask 208 is positioned such that the surface of the
mask substrate 208a opposite to the pattern surface 208c is the top, and
is supported at the outside of the frame 208d.

[0078]The photomask inspection apparatus 200 also uses laser light L21 of
193 nm in wavelength for illumination. The laser light L21 is incident on
the half mirror 201 and divided into lights in two directions. First, the
reflected illumination will be described. Laser light L22 that has passed
through the half mirror 201 travels through the lens 202a with converging
and is incident on the homogenizing optical system 203a. Laser light L23
homogenized in spatial light intensity distribution is output by the
homogenizing optical system 203a and passes through the lens 202b to be
collimated and is incident on the polarization beam splitter 204. Because
it is S waves, the laser light L23 is reflected by the polarization beam
splitter 204, travels downward, and passes through the quarter wavelength
plate 205 to be circularly polarized into laser light L24. The laser
light L24 is incident on the object lens 212, passes through the pure
water 214 and the mask substrate 208a of the mask 208, and illuminates an
observed area in the pattern surface 208c.

[0079]Next, the transmitted illumination will be described. Laser light
L25 that has been reflected by the half mirror 201 is reflected by the
mirror 211a and passes through the lens 202c to be incident on the
homogenizing optical system 203b. Laser light L26 homogenized in spatial
light intensity distribution is output by the homogenizing optical system
203b, passes through the lens 202d to be collimated, is deflected by the
mirror 211b to be incident on the condenser lens 206, and illuminates the
observed area in the pattern surface 208c of the mask 208. The condenser
lens 206 is placed under the pellicle 208b of the mask 208.

[0080]An optical system for observing an observed area in the pattern
surface 208c of the mask 208 will be described below. Laser light L24
imparted with an optical image originating from the observed area in the
pattern surface 208c illuminated by the transmitted illumination or the
reflected illumination is output upward from the object lens 212. The
laser light L24 comprises reflected light reflected by the mask 208 and
transmitted light transmitted by the mask 208. The laser light L24
reflected by the mask 208 is circularly polarized in a direction opposite
to that of the laser light L24 for illumination reflected by the
polarization beam splitter 204 and hence, passing through the quarter
wavelength plate 205 again, becomes linearly polarized waves, here P
waves. Thus, the laser light L24 reflected by the mask 208 passes through
the polarization beam splitter 204, then passes through the projection
lens 207, and strikes the two-dimensional photosensor 209. That is, an
image of the observed area in the pattern surface 208c is enlarged by the
ratio of the focal distance of the projection lens 207 to the focal
distance of the object lens 212 and enlargement projected onto the
two-dimensional photosensor 209. A CCD, a TDI, or the like is suitable as
the two-dimensional photosensor 209 like in the embodiment 1. The laser
light L24 that has passed through the mask 208 is also output from the
object lens 212 and passes through the same optical path to be incident
on the two-dimensional photosensor 209.

[0081]In the photomask inspection apparatus 200 according to the present
embodiment, the pure water 214 filling the space between the object lens
212 and the mask substrate 208a is supplied from the pure water supply
unit 215 and sucked into the pure water sucking unit 216. This scheme is
called "local fill" and has a merit that the water bath 113 in the
embodiment 1 is not necessary.

[0082]The object lens 212 is placed on the opposite side of the mask 208
from the pattern surface 208c. That is, the object lens 212 is placed
directly above the surface of the mask substrate 208a opposite to the
pattern surface 208c, and the space between the object lens 212 and the
mask substrate 208a is filled with the pure water 214. Thus, an air layer
is not formed between the pattern surface 208c and the object lens 212.
By this means, only the pure water 214 of about 1.44 in refractive index
and the mask substrate 208a of about 1.56 in refractive index are
sandwiched between the pattern surface 208c and the object lens 212.

[0083]Therefore, as described in the embodiment 1, also in the present
embodiment, the sensitivity can be raised to as high a level as when
inspected with a light source having a wavelength equal to the wavelength
of that inspection light source divided by the refractive index of
synthetic quartz. By this means, the resolution can be increased by a
factor of about 1.56, which equals the index of the synthetic quartz in
contact with the pattern surface 208c, as compared with the conventional
apparatus using an inspection light source of the same wavelength.

Embodiment 3

[0084]A photomask inspection apparatus according to embodiment 3 of the
present invention will be described with reference to FIG. 3. FIG. 3
shows the configuration of part of a photomask inspection apparatus 300
according to the present embodiment. The present embodiment differs from
the embodiment 1 in the liquid immersion part of the photomask inspection
apparatus. In FIG. 3, the optical basic configuration is omitted because
of being the same as in the embodiment 1 shown in FIG. 1. FIG. 3 shows
the configuration of the liquid immersion part of the photomask
inspection apparatus 300.

[0085]As in the embodiment 1, the photomask inspection apparatus 300
according to the present embodiment is also for inspecting a finished
mask comprising a frame 301d provided so as to surround a pattern surface
301c formed on a mask substrate 301a and a pellicle 301b applied to this.

[0086]In the present embodiment, the liquid immersion part is configured
such that pure water 305 is in contact with the back surface of the mask
substrate 301a opposite to the pattern surface 301c. The method of
supplying the pure water 305 differs from that of the photomask
inspection apparatus 100 of the embodiment 1. As shown in FIG. 3, the
pure water 305 is discharged from an outlet 304 attached to an end of a
supply tube 303 and fills the space between the top of an object lens 302
and the lower surface of the mask substrate 301a. A tray 306 is attached
surrounding the object lens 302 to receive pure water flowing down its
side, and the pure water 305 having flowed down the side of the object
lens 302 is received by the tray 306 and drained through a drain tube 307
attached to the underside of the tray 306.

[0087]The photomask inspection apparatus 300 according to the present
embodiment does not need the water bath 113 like the photomask inspection
apparatus 100 of FIG. 1. Also, the photomask inspection apparatus 300
differs from the photomask inspection apparatus 200 of FIG. 2 using the
local-fill scheme where the object lens 212 is placed above the mask 208
in that the object lens 302 is placed below the mask 301, which is also a
local-fill scheme.

[0088]In the local-fill scheme where the object lens 212 is placed above
the mask 208 as in the photomask inspection apparatus 200 of FIG. 2, in
order to retrieve pure water, pure water needs to be sucked up, and all
pure water needs to be retrieved so that the pure water does not flow out
away, but a sucking device that can reliably perform this was difficult
to realize.

[0089]In contrast, in the local-fill scheme of the photomask inspection
apparatus 300, because the object lens 302 is placed below the liquid
immersion part, the pure water 305 that has spread outside the narrow gap
of the liquid immersion part flows down naturally without actively
sucking pure water. Thus, all the pure water 305 can be retrieved only by
putting the tray 306 in place to receive it. As such, by placing the
object lens 302 below the liquid immersion part, the inspection apparatus
is simply configured.

[0090]The mounted position of the outlet 304 relative to the object lens
302 is preferably upstream in a scan direction of the mask 301. By this
means, the pure water 305 discharged from the outlet 304 fills the liquid
immersion part smoothly by the mask 301 being scanned.

[0091]FIGS. 4A and 4B illustrates an object lens 302B that differs in the
method of supplying pure water from the object lens 302 of the photomask
inspection apparatus shown in FIG. 3. FIG. 4A shows the configuration of
the object lens 302B, and FIG. 4B shows a preferable example of scan
using the object lens 302B. As shown in FIG. 4A, an last lens 321 of the
object lens 302B closest to the observed surface is flat. The space
between this last lens 321 and the back surface of the mask substrate
301a is filled with pure water. The object lens 302B has circular outlets
322a, 322b for discharging pure water near the last lens 321. The outlets
322a, 322b are placed in a mask scan direction with respect to the last
lens 321. That is, the outlets 322a, 322b are placed in parallel to the
scan direction.

[0092]Since the object lens 302B shown in FIG. 4A is provided with the two
outlets 322a, 322b, in whichever of the backward and forward scan
directions the mask moves, pure water from one of the outlets 322a, 322b
can fill the space between the last lens 321 and the mask substrate
without interruption. Pure water to be drained flows out in two
directions perpendicular to the scan direction. In order for the pure
water to flow down smoothly, gently concaved notches 323 are formed on
opposite sides of the object lens 302B. That is, the notches 323 are made
in the lens barrel of the object lens and placed along the direction
perpendicular to the scan direction. The object lens 302B shown in FIG.
4A may have attached thereto the tray 306 as attached to the object lens
302 of FIG. 3.

[0093]Usually, while the shape of the mask substrate is a square having a
side of 152 mm, the pattern area is a rectangle having a maximum size of
132 mm×104 mm. Thus, there are narrow and broad areas outside the
pattern area on the mask substrate. Thus, where the object lens 302B
having the two outlets 304 provided thereon is used, as shown in FIG. 4B,
the two outlets 304 may be arranged relative to the center of the object
lens 302B to be along the short side direction of the rectangular pattern
area 308. By this means, even when the object lens 302B is located under
a long side edge of the pattern area 308, the outlets 304 are not outside
the mask substrate 301a. Further, even when the object lens 302B is
located under a short side edge of the pattern area 308 as indicated by
the object lens 302B', the outlets 304 are not outside the mask substrate
301a. Note that arrows on sides of the object lenses 302B, 302B' indicate
the movements of the object lenses 302B, 302B' relative to the mask
substrate 301a and in reality, the object lens is fixed while the mask is
moved in scan.

[0094]The structure of an object lens 330 of another structure suitable
for the photomask inspection apparatus 300 of the present invention will
be described with reference to FIGS. 5A and 5B. FIG. 5A is a perspective
view of the object lens 330, and FIG. 5B is a sectional view thereof. The
object lens 330 is provided at two places with outlets 332a, 332b for
discharging pure water like the object lens 302B of FIG. 3, but a pure
water inlet 333 is formed like a ring surrounding the two outlets 332a,
332b. By this means, all pure water discharged from the outlets 332a,
332b is sucked through the inlet 333 so as not to flow outside the object
lens 330. Hence, the tray 306 as shown in FIG. 3 is not necessary.

[0095]The structure of an object lens 340 of another structure will be
described with reference to FIGS. 6A and 6B. FIG. 6A is a perspective
view of the object lens 340, and FIG. 6B is a sectional view thereof. The
object lens 340 is similar to the object lens 330 of FIG. 5A but differs
in that a ring-shaped air outlet 345 is provided around a pure water
inlet 343 as shown in FIG. 6A. Further, a tube connection opening 346 is
made in the side of the object lens 340, and air is discharged through
the air outlet 345. By this means, part of pure water not sucked into the
inlet 343 and flowing out can be prevented from flowing outside the
object lens 340.

[0096]Next, the thickness of pure water in the photomask inspection
apparatus of the present invention will be described using FIGS. 7A, 7B
and 7C. FIGS. 7A, 7B and 7C show only the liquid immersion part and its
neighborhood of the photomask inspection apparatus 100 of FIG. 1. As
shown in FIG. 7A, let tmask be the thickness of the mask substrate 108a
and twater be the thickness of the pure water 114. In the present
invention, the distance between the mask 108 and the water bath 113 is
adjusted so as to keep tmask+twater=7.00 mm not depending on the
thickness of the mask substrate 108a. For example, when the thickness of
the mask substrate 108a is at a standard of 6.35 mm, the thickness of the
pure water 114 is at 0.65 mm. When the thickness of the mask substrate
108a is at 6.45 mm, greater than the standard, the thickness of the pure
water 114 is finely adjusted to 0.55 mm.

[0097]In order to keep the sum of the thickness of the mask substrate 108a
and the thickness of the pure water 114 at a constant value (here 7.00
mm) not depending on the mask, the photomask inspection apparatus of the
present invention comprises an adjustment mechanism for adjusting the
thickness of the pure water 114 according to the thickness of the mask
substrate 108a as shown in FIG. 7B or 7C.

[0098]The adjustment mechanism shown in FIG. 7B, using a measurement
semiconductor laser 120 attached to the edge of the water bath 113,
finely adjusts the placement height of the mask substrate 108a such that
the distance from the semiconductor laser 120 to the pattern surface 108c
is constant. The arrow going down from the semiconductor laser 120
indicates laser light, and the semiconductor laser 120 (its receiving
portion being omitted from the figure) receives light reflected by the
upper surface of the mask substrate 108a to measure the distance thereto.
Then, the height is adjusted by adjusting the amount of the pure water
114 in the water bath 113.

[0099]When designing the lens and the like in the object lens 112, also
the mask substrate 108a placed in the optical path to the pattern surface
108c, an image plane, needs to be considered as a transparent plane
parallel substrate. Since design is usually performed assuming that the
thickness of the mask substrate 108a is constant (usually 6.35 mm), if
the thickness varies, the distortion of the image increases, strictly
speaking, while the focus can be readjusted. However, according to the
present embodiment, the sum of the optical distance in the mask substrate
108a made of synthetic quartz and the optical distance of the pure water
114 can be kept substantially constant not depending on the thickness of
the mask substrate 108a. By this means, the distortion of an enlarged
image of the pattern surface 108c can be reduced to a negligible level.

[0100]An adjustment mechanism shown in FIG. 7c may be provided as another
mechanism for keeping the sum of the optical distance in the mask
substrate 108a and the optical distance of the pure water 114. The
adjustment mechanism shown in FIG. 7c is provided on the side of a lens
barrel containing the lenses and the like of an object lens 112B. As
shown in FIG. 7c, a semiconductor laser 120B is attached to the object
lens 112B. Laser light L10 output from the semiconductor laser 120B
travels through the condenser lens 121 with converging and passes through
a triangle prism 122a made of sapphire to be deflected. The triangle
prism 122a is in contact with the pure water 114, and the laser light
travels through the pure water 114 and the mask substrate 108a and
strikes the pattern surface 108c at a large incident angle. Hence, all
laser light L11 is reflected by the pattern surface 108c, travels through
the pure water 114 again, and is incident on a triangle prism 122b to be
deflected into laser light L12, which strikes a two-division sensor 123.
Two signals from the two-division sensor 123 are led to a controller (not
shown) through a signal line 124.

[0101]According to the example of FIG. 7c, when the sum of the thickness
of the mask substrate 108a and the thickness of the pure water 114
varies, the position at which the laser light L12 strikes the
two-division sensor 123 shifts to the right and left in FIG. 7c, which is
detected in the form of variation in balance between the two signals.
Therefore, the position of the mask substrate 108a may be finely adjusted
vertically such that the signal balance is constant not depending on the
thickness of the mask substrate 108a.

[0102]In the above example, the placement height of the mask substrate
108a is finely adjusted such that the sum of the thickness of the mask
substrate 108a and the thickness of the pure water 114 is constant not
depending on the mask, but in addition, the distortion can be reduced. To
be specific, if the thickness of the mask substrate 108a is greater than
the standard, the placement height of the mask substrate 108a is adjusted
to be slightly lower. If the thickness of the mask substrate 108a is less
than the standard, the placement height of the mask substrate 108a is
adjusted to be slightly higher. As a result, the sum of the optical
distance in the mask substrate 108a and the optical distance in the pure
water 114 can be made constant not depending on the thickness of the mask
substrate 108a.

[0103]For example, when the thickness of the mask substrate 108a is at
6.40 mm, greater by 0.05 mm than the standard of 6.35 mm, the placement
height of the mask substrate 108a may be lowered by 0.004 mm. When the
thickness of the mask substrate 108a is at 6.30 mm, less by 0.05 mm than
the standard of 6.35 mm, the placement height of the mask substrate 108a
may be raised by 0.004 mm.

[0104]The optical distance (distance over which light travels in vacuum to
be inphase) in synthetic quartz 0.05 mm thick is 0.078 mm because the
refractive index of synthetic quartz is about 1.5608 at a wavelength of
193 nm. Meanwhile, the thickness of pure water for which the optical
distance is 0.078 mm is 0.054 mm because the refractive index of
synthetic quartz is about 1.436 at a wavelength of 193 nm. Therefore, if
pure water is simply made 0.050 mm thinner so as to keep the sum of the
thicknesses of the two layers constant, the pure water is still about
0.004 mm thicker, strictly speaking.

[0105]As such, in the present invention, in order to further reduce the
distortion due to variation in the thickness of the mask substrate, the
placement height of the mask substrate may be finely adjusted so as to
keep the optical distance of laser light passing through two layers of
the mask substrate and immersion liquid constant. As described above, the
pattern surface 108c is provided on the upper side, and the object lens
112 is placed below the water bath 113, and further the measurement unit
for measuring the height of the pattern surface in the water bath 113 is
provided. By this means, the optical path length can be adjusted to focus
laser light on the pattern surface, thus further improving detection
sensitivity. The measurement unit may have a configuration other than the
above.

[0106]In the embodiment having been described, pure water is used as
liquid for the liquid immersion, but instead of pure water,
fluorine-based liquid (Fluorinert) may be used. The following table shows
examples of usable fluorine-based liquid and their properties.

[0107]Next, a dead space that cannot be inspected by the photomask
inspection apparatus will be described with reference to FIGS. 8A and 8B.
FIG. 8A shows the dead space for when a conventional photomask inspection
apparatus is used, and FIG. 8B shows the dead space for when the
photomask inspection apparatus of the present invention is used. As shown
in FIG. 8A, in the conventional photomask inspection apparatus, an object
lens 400a is placed on the pattern surface 402a side of a mask substrate
401a. Hence, if the numerical aperture NA of the object lens 400a is very
large, illumination light will be blocked by a spacer 403a supporting a
pellicle 404a.

[0108]As shown in FIG. 8B, in the present invention, an object lens 400b
is placed on the back surface side of a mask substrate 401b. Hence, if
the NA of the object lens 400b is very large, an inspectable area of a
pattern surface 402b on the mask substrate 401b will not be reduced by
the blocking of a spacer 403b supporting a pellicle 404b. That is,
according to the present invention, the dead space of the pattern surface
402b will be reduced. In, e.g., the photomask inspection apparatus 200 of
FIG. 2, the entire surface of a mask can be inspected with use of
reflected illumination. If the numerical aperture of the condenser lenses
106, 206 is smaller than that of the object lens 112, 212, the dead space
can be reduced.

Embodiment 4

[0109]A photomask inspection apparatus according to embodiment 4 of the
present invention will be described with reference to FIG. 9. FIG. 9
shows the configuration of a photomask inspection apparatus 500 according
to the embodiment 4. While the embodiments 1 to 3 are photomask
inspection apparatuses for usual masks with a pellicle for use in ArF or
krF lithography, the present embodiment is a photomask inspection
apparatus for imprint masks without a pellicle. For masks made of
transparent optical material such as quartz from among various imprint
masks, the photomask inspection apparatus of the embodiment 1 can be used
as it is. As shown in FIG. 9, the photomask inspection apparatus 500 has
basically the same configuration as the photomask inspection apparatus
100 for ArF or KrF masks of FIG. 1.

[0110]In the present embodiment, laser light L51 of 193 nm in wavelength
from the inspection light source 530 is used for illumination. The laser
light L51 is incident on the half mirror 501 and divided into lights in
two directions. Laser light L52 that has passed through the half mirror
501 travels through the lens 502a with converging and is incident on the
homogenizing optical system 503a. Laser light L53 homogenized in spatial
light intensity distribution is output by the homogenizing optical system
503a and passes through the lens 502b to be collimated and is deflected
by the mirror 511b to be incident on the condenser lens 506, which
condenses it into laser light L54. The laser light L54 illuminates an
observed area in the pattern surface 508c, the upper surface, of an
imprint mask 508.

[0111]Meanwhile, laser light L55 that has been reflected by the half
mirror 501 is reflected by the mirror 511a and passes through the lens
502c to be incident on the homogenizing optical system 503b. Laser light
L56 homogenized in spatial light intensity distribution is output by the
homogenizing optical system 503b, passes through the lens 502d to be
collimated, and is incident on the polarization beam splitter 504.
Because it is S waves, laser light L57 is reflected by the polarization
beam splitter 504 and travels upward as indicated by laser light L58. The
laser light L58 passes through the quarter wavelength plate 505 to be
circularly polarized into laser light L59. The laser light L59 is
incident on the object lens 512 fixed in the middle of the water bath 513
and passes through the pure water 514 and an imprint mask 508 and
illuminates a pattern surface 508c. Since being made of synthetic quartz,
the imprint mask 508 of the present embodiment well transmits the laser
light of 193 nm in wavelength.

[0112]Laser light L59 imparted with an optical image originating from an
observed area in the pattern surface 508c illuminated by the transmitted
illumination or the reflected illumination is output downward from the
object lens 512. This laser light L59 comprises reflected light reflected
by the mask 508 and transmitted light transmitted by the mask 508. The
laser light L59 reflected by the mask 508 is circularly polarized in a
direction opposite to that of the laser light L59 incident on the mask
508 and hence, passing through the quarter wavelength plate 505 again,
becomes linearly polarized waves, here P waves. Thus, the laser light L59
reflected by the mask 508 passes through the polarization beam splitter
504. As a result, the reflected light passes through the projection lens
507 and strikes the two-dimensional photosensor 509. That is, an image of
the observed area in the pattern surface 508c is enlarged by the ratio of
the focal distance of the projection lens 507 to the focal distance of
the object lens 512 and enlargement projected onto the two-dimensional
photosensor 509. A CCD, a TDI, or the like is suitable as the
two-dimensional photosensor 509. The laser light L59 that has passed
through the mask 508 is also output from the object lens 512 and passes
through the same optical path to be incident on the two-dimensional
photosensor 509.

[0113]As such, also for an imprint mask with a fine size pattern, the
photomask inspection apparatus of the present invention can be used as
long as its substrate is transparent. Hence, defect inspection can be
conveniently performed with high sensitivity without using a microscope
that needs vacuum such as a SEM (Scanning Electron Microscope).

[0114]In the conventional photomask inspection apparatus, since observing
from above the top of a pattern, measuring pattern lengths results in
measuring the sizes of the top. Because the top is usually round at its
edges, it is difficult to accurately measure. However, in the present
invention, the pattern surface on a mask substrate is observed from the
back surface side of the mask substrate. Hence, the bottom shape of the
pattern can be accurately observed. As a result, the pattern lengths can
be accurately measured. Therefore, the photomask inspection apparatus of
the present invention can also be used in place of a length measuring
SEM.

Embodiment 5

[0115]A photomask inspection apparatus according to embodiment 5 of the
present invention will be described with reference to FIG. 10. FIG. 10
shows the configuration of a photomask inspection apparatus 600 according
to the present embodiment. The photomask inspection apparatus 600
according to the present embodiment also inspects imprint masks like the
embodiment 4.

[0116]The photomask inspection apparatus 600 also uses laser light L61 of
193 nm in wavelength from the inspection light source 630 for
illumination. The laser light L61 is incident on the half mirror 601 and
divided into lights in two directions. Laser light L62 that has passed
through the half mirror 601 travels through the lens 602a with converging
and is incident on the homogenizing optical system 603a. Laser light L63
homogenized in spatial light intensity distribution is output by the
homogenizing optical system 603a, passes through the lens 602b to be
collimated, and is incident on the polarization beam splitter 604.
Because it is S waves, laser light L63 is reflected by the polarization
beam splitter 604 and travels downward and passes through the quarter
wavelength plate 605 to be circularly polarized into laser light L64. The
laser light L64 is incident on the object lens 612, passes through the
pure water 614 and an imprint mask 608, and illuminates a pattern surface
608c formed on the opposite side.

[0117]Meanwhile, laser light L65 that has been reflected by the half
mirror 601 is reflected by the mirror 611a and passes through the lens
602c to be incident on the homogenizing optical system 603b. Laser light
L66 homogenized in spatial light intensity distribution is output by the
homogenizing optical system 603b, passes through the lens 602d to be
collimated, is deflected by the mirror 611b to be incident on the
condenser lens 606, and illuminates the pattern surface 608c of the
imprint mask 608.

[0118]Laser light L64 imparted with an optical image originating from an
observed area in the pattern surface 608c illuminated by the transmitted
illumination or the reflected illumination is output upward from the
object lens 612. The laser light L64 comprises reflected light reflected
by the mask 608 and transmitted light transmitted by the mask 608. The
laser light L64 reflected by the mask 608 is circularly polarized in a
direction opposite to that of the laser light L64 and hence, passing
through the quarter wavelength plate 605 again, becomes linearly
polarized waves, here P waves. Thus, the laser light L64 reflected by the
mask 608 passes through the polarization beam splitter 604 and passes
through the projection lens 607 and strikes the two-dimensional
photosensor 609. Thereby, an image of the observed area in the pattern
surface 608c is enlarged and projected onto the two-dimensional
photosensor 609. A CCD, a TDI, or the like is suitable as the
two-dimensional photosensor 609 like in the above embodiments. The laser
light L64 that has passed through the mask 608 is also output from the
object lens 612 and passes through the same optical path to be incident
on the two-dimensional photosensor 609.

[0119]The feature of the present embodiment is that the pure water 614
filling the space between the object lens 612 and the imprint mask 608 is
supplied from the pure water supply unit (not shown) and sucked into the
pure water sucking unit (not shown), which are attached to the object
lens 612, like with the object lens 330 of FIG. 5A or the object lens 340
of FIG. 6A. The photomask inspection apparatus 600 of the present
embodiment features the use of the local-fill scheme. Consequently, the
pure water 614 does not fall off the imprint mask 608, and hence the
object lens 612 can be placed above the imprint mask 608. That is, the
pattern surface 608c of the imprint mask 608 faces downward. This
direction is the same as the direction in which to imprint a wafer in an
imprint exposure apparatus. Thus, when the photomask inspection apparatus
600 of the present embodiment is used to regularly inspect imprint masks
used in an imprint exposure apparatus, the imprint masks need not be
inverted. Therefore, with the photomask inspection apparatus 600
incorporated in an imprint exposure apparatus, quick mask inspection can
be performed.

[0120]The imprint mask 608 in the present embodiment is four times the
size of the usual imprint mask 508 of FIG. 9 and is of the same size as a
usual KrF mask or ArF mask, called a 6-inch mask. The inspection of this
imprint mask will be described with reference to FIG. 11. As shown in
FIG. 11, a pattern area 615 covers only the middle of the imprint mask
608. Hence, when observing the edge of the pattern area 615 with the
object lens 612 having an outlet 616 for the pure water 614, that is,
when the last lens 618 of the object lens 612 is located over an edge of
the pattern area 615, the outlet 616 and the inlet 617 is not outside the
imprint mask 608. Therefore, in the present embodiment, the object lens
612 is placed above the imprint mask 608, and the local-fill scheme can
be used.

Embodiment 6

[0121]A photomask inspection apparatus according to embodiment 6 of the
present invention will be described with reference to FIG. 12. FIG. 12
shows the configuration of a photomask inspection apparatus 700 according
to the present embodiment. The photomask inspection apparatus 700
inspects EUV masks. In the photomask inspection apparatus 700, laser
light L71 of 193 nm in wavelength for illumination is S waves. Hence, the
laser light L71 from the inspection light source 730 is reflected by the
polarization beam splitter 705, travels downward, passes through the
quarter wavelength plate 706 to be circularly polarized, then passes
through the object lens 707, and illuminates a pattern surface 702 of an
EUV mask 701. Laser light L72 imparted with an optical image originating
from the illuminated pattern surface 702 travels upward through the
object lens 707 and passes through the quarter wavelength plate 706 again
to become here P waves. Thus, the laser light L72 is transmitted by the
polarization beam splitter 705 and enlargement projected by the
projection lens 708 onto the two-dimensional photosensor 709. The laser
light reflected by the mask 701 passes through the quarter wavelength
plate 706 to become P polarized like the above embodiments. Thus, light
use efficiency can be improved.

[0122]In the present embodiment, the EUV mask 701 is placed in a water
bath 710 containing high-refractive-index liquid 711. Thus, the space
between the pattern surface 702 of the EUV mask 701 and the object lens
707 is filled with the high-refractive-index liquid 711. In the present
embodiment, the pattern surface 702 having recesses and raised portions
is in contact with the high-refractive-index liquid 711. In order to
allow the high-refractive-index liquid 711 to permeate into the recesses,
the liquid immersion optical system with the water bath 710 is used, and
the EUV mask 701 is immersed in the high-refractive-index liquid 711 in
the water bath 710.

[0123]A liquid of about 1.64 in refractive index is used as the
high-refractive-index liquid 711. As a result, substantially the same
level of resolution is obtained as in the case of inspection with an
inspection light source of 118 nm in wavelength. For example, high-purity
carbon-hydrogen-based liquid developed by JSR Corp. is suitable as the
high-refractive-index liquid 711, but instead of the
high-refractive-index liquid 711, pure water may be used in this
embodiment as well.

[0124]As described above, with the photomask inspection apparatus
according to the present invention, the actual wavelength of the
inspection light source is about 124 nm when laser light of 193 nm in
wavelength is used as the light source. Hence, defects and the like can
be detected with sensitivity about 1.56 times as high as with a
conventional apparatus. Since the limit size of detectable defects is
said to be about one fourth of the inspection light wavelength, the limit
size becomes about 31 nm. That is, for quadruple-size masks, transferred
defects on wafers of sizes down to about 8 nm can be covered. Thus, not
only a 22 nm generation of lithography but also 16 nm and 11 nm
generations can be covered. For imprint masks, i.e., equal-size masks, a
32 nm generation can be covered.

[0125]In the present invention, when retrieving an inspected mask, pure
water sticking to the mask substrate needs to be removed. Blowing off by
dry air or sucking with a vacuum sucking device can be used, or the back
surface of the mask substrate may be wiped with cylindrical sponge made
of non-dusting material, and thereby pure water can be easily removed
from the mask.

[0126]In order to enable highly accurate inspection with the photomask
inspection apparatus of the present invention, synthetic quartz for the
mask substrate is preferably material of high quality used for lenses of
exposure apparatuses, that is, quartz with as small variation in
refractive index as possible.

[0127]Moreover, the following modification may be made to the mask. Where
pure water is used as liquid for liquid immersion, because there is a
slight difference in refractive index between the pure water and quartz
of the mask substrate, antireflection coating may be coated on the back
surface of the mask substrate. Further, where the mask 108 is moved in
the water bath 113 as shown in FIG. 1, hydrophobic coating may be coated
around the mask substrate 108a so that water hardly sticks thereto.

[0128]Although description has been made taking a UV exposure mask or an
imprint mask as an example, the present invention is not limited to this.
That is, the photomask inspection apparatus can inspect masks for use in
exposure for forming patterns. Thus, the masks in the present invention
include reticles as well. Although in the above description light from
the inspection light source is divided into two lights for transmitted
illumination light and reflected illumination light, the present
invention is not limited to this. For example, the photomask inspection
apparatus may have a transmitted illumination light source and a
reflected illumination light source. Further, the embodiments may be
combined as needed. Yet further, in order to pick up transmitted images
or reflected images, a shutter may be inserted in each optical path. When
transmitted images are picked up, a shutter is inserted in the optical
path for the reflected illumination light, and when reflected images are
picked up, a shutter is inserted in the optical path for the transmitted
illumination light.

[0129]The main effects of the present invention will be described below.
In the photomask inspection apparatus of the present invention, applying
the liquid immersion technique, the space between the pattern surface of
the mask and the last lens of the object lens closest to the mask
substrate is filled with liquid, expelling an air layer. By this means,
with the simple configuration, resolution and thus defect detection
sensitivity can be improved. Further, the transmitted illumination from
the pattern surface side enables the inspection of masks with a pellicle.
As shown in the embodiments 1 to 5, the object lens is preferably placed
on the back surface (non-pattern surface) side of the mask substrate, the
opposite side thereof from the pattern surface. By this means, the liquid
can be prevented from sticking to the pattern surface.

[0130]An image from the observed area of the pattern surface does not pass
through the object lens without passing through the mask substrate.
Because the mask substrate of ArF masks or KrF masks is made of
transparent synthetic quartz, the pattern surface can be observed from
the back surface side. Thus, the liquid immersion optical system where
the space between the back surface and the last lens of the object lens
is filled with liquid can be used. Further, since the back surface to
contact the liquid is extremely flat, sticking liquid is extremely easy
to remove, and it does not occur that the liquid is difficult to remove
from the mask substrate after inspection.

[0131]As such, it is a feature of the present invention that the liquid
immersion part is the flat back surface. The reason is as follows. If the
pattern surface is immersed in liquid, the liquid will flow along the
pattern surface having recesses and raised portions, and particularly at
high pattern-density part, it is difficult for the liquid to go into
between the raised portions, and thus it is difficult to achieve complete
liquid immersion.

[0132]In the present invention, the term object lens refers to a group of
lenses arranged on the mask pattern side in an optical system which
enlargement projects an image of a to-be-inspected area in a mask pattern
onto a two-dimensional photosensor or the like for detecting an image.
The enlargement projecting optical system comprises an object lens and at
least one more group of lenses or lens (herein called a projection lens).
The focal distance of the projection lens is longer than that of the
object lens, and thus the image is enlargement projected.

[0133]Usually, the thickness of the mask substrate is specified as 6.35
mm, but in reality, there is a variation of about ±0.1 mm. Hence, an
image of the observed area in the pattern surface may be distorted. The
reason is as follows. Each lens in the object lens used in the present
invention is designed assuming that a 6.35 mm thick quartz-made parallel
plate is included in the projection optical system. Hence, when the
thickness of the mask substrate varies, focusing can be achieved by
adjusting the distance between the object lens and the observed area, but
it is difficult to design each lens so as to control distortion to a
negligible level, about several nm or less, over the entire range of
variations in thickness.

[0134]Accordingly, as described above, the adjustment mechanism is
provided to adjust the thickness of water filling the space between the
mask substrate and the last lens of the object lens at least for each
mask to be inspected. By this means, even if there is variation in the
thickness of the mask substrate, the sum of the thickness of the mask
substrate and the thickness of water can be kept substantially constant.

[0135]The refractive index of quartz of the mask substrate is about 1.56
at a wavelength of 193 nm, and the refractive index of water is about
1.44. These are much greater than the 1.0 of air. Thus, the total optical
distance of the two layers can be kept substantially the same with
variation in the thickness of the mask substrate cancelled out by the
other. Therefore, observation at the focal position can be performed,
thus improving detection sensitivity.

Embodiment 7

[0136]A photomask inspection apparatus according to embodiment 7 of the
present invention will be described with reference to FIGS. 16 to 18.
FIG. 16 shows the configuration of a photomask inspection apparatus 100
according to the present embodiment. FIGS. 17A and 17B show the
configuration of a liquid immersion object lens 112A of the present
embodiment. FIG. 17A is a view of the object lens 112A as seen from above
(from the last lens 115 side), and FIG. 17B is a sectional view thereof.
FIGS. 18A and 18B illustrate the relative position of the object lens
112A to a pattern area 108e of a mask. FIG. 18A is a view of the mask 108
as seen from above, and FIG. 18B is a side view thereof. The photomask
inspection apparatus of the present embodiment differs from the photomask
inspection apparatus 100 shown in FIG. 1 of the embodiment 1 in the
structure of the object lens 112A. In FIG. 16, the same reference
numerals indicate the same components as in the photomask inspection
apparatus of FIG. 1 with description thereof being omitted.

[0137]As described above, the inventors applied the liquid immersion
technique to photomask inspection apparatuses and invented a liquid
immersion photomask inspection apparatus whose sensitivity can be
improved by a factor of about 1.5. As opposed to an exposure apparatus to
which the liquid immersion technique has been applied, the pattern
surface of a mask is covered by a transparent thin film called a
pellicle, and hence the pattern surface cannot be directly in contact
with liquid immersion water such as pure water. Accordingly, the
inventors invented a liquid immersion photomask inspection apparatus
configured to observe the pattern surface from the back surface side of
the mask substrate so that the liquid immersion technique can be applied.

[0138]However, there is the following task with the liquid immersion
photomask inspection apparatus. Where as shown in FIGS. 20A and 20B the
object lens 112 is placed under the mask 108, in order to prevent claws
of an arm supporting the mask 108 from interfering with the object lens
112, the maximum allowable pattern surface 108c cannot entirely be
inspected.

[0139]As shown in FIGS. 20A and 20B, the maximum size of the pattern area
108e of a 6-inch mask of 152 mm square is 132 mm×104 mm. Hence,
when inspecting an edge of the pattern area 108e, the body (if
cylindrical, its outer diameter) of the liquid immersion object lens 112
is partly outside the 152-mm-square mask 108. Meanwhile, the liquid
immersion water fills the space between the mask substrate 108a and the
liquid immersion object lens 112, but since the space is usually as thin
as about 1 mm, the claws of the arm cannot be inserted into the space.
Thus, there was the problem that in the type of liquid immersion
photomask inspection apparatus that the object lens 112 is placed under
the mask 108, the inspectable area of the pattern area 108e is about
several tens mm narrower than the maximum allowable size.

[0140]One reason why when the object lens 112 is located near an edge of
the pattern area 108e for observation, its body is partly outside the
mask 108 is that usually the outer diameters of a plurality of lenses
forming the object lens 112 used in the photomask inspection apparatus
are larger than that of the last lens. For example, when the outer
diameter of the last lens is about 20 mm, lenses of about 50 mm in outer
diameter are used inside. Another reason is that while in order to
observe at a high magnification of 100 or greater, the focal distance of
the object lens 112 used in the photomask inspection apparatus needs to
be as very short as 2 to 3 mm, work distance needs to be about 7 to 8 mm,
greater than the focal distance. As a result, the object lens as shown in
FIGS. 20A and 20B, of about 50 to 60 mm in outer diameter are used
inside.

[0141]In order to solve the problem, the inventors invented the object
lens 112A as shown in FIG. 16. As shown in FIG. 16, a photomask
inspection apparatus 100, as in the embodiment 1, comprises an inspection
light source 130, a half mirror 101, lenses 102a to 102d, homogenizing
optical systems 103a, 103b, a polarization beam splitter 104, a quarter
wavelength plate 105, a condenser lens 106, a projection lens 107, a
two-dimensional photosensor 109, mirrors 111a, 111b, the object lens
112A, a water bath 113, and pure water 114. The photomask inspection
apparatus 100 uses laser light L01 of 193 nm in wavelength from the
inspection light source 130 for illumination. The laser light L01 is
incident on the half mirror 101 and divided into lights in two
directions.

[0142]First, the transmitted illumination will be described. Laser light
L02 that has passed through the half mirror 101 travels through the lens
102a with converging and is incident on the homogenizing optical system
103a. Laser light L03 homogenized in spatial light intensity distribution
is output by the homogenizing optical system 103a and passes through the
lens 102b to be collimated and is deflected by the mirror 111b to be
incident on the condenser lens 106, which condenses it into laser light
L04. The laser light L04 illuminates an observed area in the pattern
surface 108c of the mask 108. The condenser lens 106 is placed directly
above the pellicle 108b of the mask 108.

[0143]Next, the reflected illumination will be described. Laser light L05
that has been reflected by the half mirror 101 is reflected by the mirror
111a and passes through the lens 102c to be incident on the homogenizing
optical system 103b. Laser light L06 homogenized in spatial light
intensity distribution is output by the homogenizing optical system 103b,
passes through the lens 102d to be collimated, and is incident on the
polarization beam splitter 104. Because it is S waves, laser light L07 is
reflected by the polarization beam splitter 104 and travels upward as
indicated by laser light L08. The laser light L08 passes through the
quarter wavelength plate 105 to be circularly polarized into laser light
L09. The laser light L09 is incident on the liquid immersion object lens
112A fixed in the middle of the water bath 113 and passes through the
pure water 114 and the mask substrate 108a of the mask 108 and
illuminates the pattern surface 108c from below.

[0144]An optical system for observing an observed area in the pattern
surface 108c of the mask 108 will be described below. Laser light L09
imparted with an optical image originating from an observed area in the
pattern surface 108c illuminated by the transmitted illumination or the
reflected illumination is output downward from the object lens 112A,
which laser light contains optical information in the form of a spatial
light intensity distribution. The laser light L09 is circularly polarized
in an opposite direction and hence, passing through the quarter
wavelength plate 105 again, becomes here P waves and passes through the
polarization beam splitter 104. As a result, the light passes through the
projection lens 107 and strikes the two-dimensional photosensor 109. That
is, an image of the observed area in the pattern surface 108c is enlarged
by the ratio of the focal distance of the projection lens 107 to the
focal distance of the liquid immersion object lens 112A and enlargement
projected onto the two-dimensional photosensor 109.

[0145]The focal distance of the liquid immersion object lens 112A is about
3 mm. Meanwhile, the focal distance of the projection lens 107 is about
300 mm. As a result, an image of the observed area of the pattern surface
108c is enlarged by a factor of about 100 and projected onto the
two-dimensional photosensor 109. A CCD, a TDI, or the like is suitable as
the two-dimensional photosensor 109.

[0146]In the photomask inspection apparatus 100, because the liquid
immersion object lens 112A is placed under the mask 108, and the pure
water 114 as liquid immersion water fills the space between the mask
substrate 108a and the liquid immersion object lens 112A during
inspection. By this means, the NA of the liquid immersion object lens
112A is as high as 1 and sensitivity is improved by a factor of about 1.5
over a conventional photomask inspection apparatus with light of the same
wavelength.

[0147]The structure of the liquid immersion object lens 112A will be
described in detail with reference to FIG. 17A. The object lens 112A
comprises a body 112a, an last lens 115, pure water supply ports 116a,
116b, first pure water sucking ports 117a, 117b, a second pure water
sucking port 118, dry air outlets 119a, 119b, and steps 140.

[0148]The body 112a of the liquid immersion object lens 112A is
substantially cylindrical. The last lens 115 is provided in the center of
the top of the body 112a. The pure water supply ports 116a, 116b are
formed respectively upstream and downstream in the mask scan direction of
the last lens 115. The pure water 114 is supplied from the pure water
supply port 116a or 116b. Which of the pure water supply ports 116a, 116b
supplies the pure water is determined by the scan direction of the mask
108 during inspection. That is, the pure water 114 is supplied from the
pure water supply port 116a or 116b upstream in the scan direction of the
mask 108 such that supplied pure water covers the entire face of the last
lens 115 during the scan of the mask 108.

[0149]The first pure water sucking ports 117a, 117b are provided
respectively upstream and downstream in the scan direction of the pure
water supply ports 116a, 116b. The pure water 114 supplied from the pure
water supply port 116a or 116b is sucked into either the first pure water
sucking port 117a or 117b. The first pure water sucking ports 117a, 117b
are connected to a vacuum sucking device (not shown), which operates all
the time during inspection.

[0150]The second pure water sucking port 118 is formed outward of the
first pure water sucking ports 117a, 117b so as to surround them. If the
pure water 114 supplied could not be completely retrieved by the first
pure water sucking port 117a or 117b, pure water that could not be sucked
is sucked into the second pure water sucking port 118, which has a large
opening surrounding them.

[0151]The dry air outlets 119a, 119b are formed outward of the second pure
water sucking port 118 upstream and downstream in the scan direction. If
pure water sticks to the mask substrate 108a and is hard for even the
second pure water sucking port 118 to suck in, by the dry air outlets
119a, 119b ejecting dry air, pure water sticking to the mask substrate
108a is blown off. The blown-off pure water is sucked into the second
pure water sucking port 118.

[0152]As shown in FIG. 17B, the steps 140 are formed in the top (the last
lens 115 side) of the body 112a on opposite sides in a direction
orthogonal to the scan direction. The body 112a of the object lens 112A
has a shape where the steps are made by cut in the area of the top
excluding the second pure water sucking port 118 and the dry air outlets
119a, 119b as well as the last lens 115. The steps are a main feature of
the present embodiment, and with the steps 140, the entire pattern area
108e of the mask can be thoroughly inspected as described below.

[0153]The height of the steps 140 corresponds to the thickness of claws
150a, 150b, 150c, and 150d of the arm described later. The height of the
step 140 can be, e.g., about 4 to 5 mm. The relative relationship between
the mask substrate 108a of the mask 108 and the object lens 112A will be
described with reference to FIGS. 18A and 18B. FIGS. 18A and 18B show the
case where the lower left edge of the pattern area 108e is observed with
the liquid immersion object lens 112A.

[0154]As shown in FIGS. 18A and 18B, the mask 108 is supported at four
corners of the mask substrate 108a by the claws 150a, 150b, 150c, and
150d of the arm from below. As shown in FIG. 18A, when the lower left
edge of the pattern area 108e is observed, the body 112a of the liquid
immersion object lens 112A is partly outside the mask 108 as seen from
above. However, as seen from side, the claw 150c of the arm is received
in the step 140 made in the top of the liquid immersion object lens 112A.
Likewise, when the upper left edge, upper right edge, and lower right
edge of the pattern area 108e are observed, the claws 150a, 150b, and
150d of the arm are received in the steps 140 of the liquid immersion
object lens 112A. Hence, the object lens 112A does not interfere with the
claws 150 of the arm, and thus the edges of the pattern area 108e can
also be observed.

[0155]The outer diameter of the liquid immersion object lens 112A of the
present invention will be described quantitatively below with reference
to FIG. 19. In FIG. 19, only the last lens 115 of the liquid immersion
object lens 112A is shown. Let nm be the refractive index of the mask
substrate 108a and θm be the angle (subtended angle) of a
diffracted ray D1 originating from the observed point and incident on the
outermost circumference of the last lens 115 relative to the optical
axis. Then, the numerical aperture NA, a characteristic that affects the
resolution, of the liquid immersion object lens 112A is expressed by the
equation (1):

NA=nm×sin θm. (1)

[0156]The diffracted ray D1 is refracted when travelling from the mask
substrate 108a into the pure water 114. Let nw be the refractive index of
the pure water 114 and θw be the refracting angle. Then, the
equation (2) holds true:

nm×sin θm=nw×sin θw. (2)

[0157]With the angles θm and θw, the thickness tm of the mask
substrate 108a, and the thickness tw of the pure water 114, the minimum
radius R necessary for the last lens 115 is obtained from the equation
(3):

R=tm×tan θm+tw×tan θw. (3)

[0158]For example, if a liquid immersion object lens 112A having an NA of
1.0 is used, by substituting the refractive index of the mask substrate
108a (synthetic quartz) at a wavelength of 193 nm=1.56077, the refractive
index of the pure water 114=1.436, the thickness tm of the mask substrate
108a=6.35 mm, and the thickness tw of the pure water 114=1.0 mm, R=6.27
mm is obtained. This is the minimum radius necessary for the last lens
115. There needs to be a margin of about 2 mm around the last lens 115
for lens support. Thus, considering the outer size of the body around the
last lens 115 (indicated by X in FIG. 19, part that has to be excluded
from the steps), the value of R+X needs to be about 9 mm or greater. This
length R+X is much smaller than the width of 24 mm outside the pattern
area 108e of the mask 108 shown in FIGS. 18A and 18B. Therefore, where
the object lens 112A is placed below the mask 108, the claws 150a, 150c
of the arm can be located between the step 140 and the mask substrate
108a.

[0159]Thus, in the photomask inspection apparatus 100, the scan direction
of the mask 108 is in such a direction that the step 140 of the liquid
immersion object lens 112A is located under the part of 24 mm in width
outside the pattern area 108e as indicated by an arrow in FIG. 18A. On
the other hand, if the mask is scanned in such a direction that the step
of the liquid immersion object lens 112A is located under the part of 10
mm in width outside the pattern area 108e (that is, a left-right
direction in FIG. 18A), the step will go outside the mask 108 and there
will be no space for the claws of the arm to be inserted under the mask
substrate 108a.

[0160]As described above, in the present embodiment, because the steps 140
are made in the body 112a of the object lens 112A, when the liquid
immersion object lens 112A comes into a position where to observe an edge
of the pattern area 108e, the claws 150a, 150b, 150c, 150d of the arm can
be inserted into the steps 140 with supporting the mask 108. Therefore,
in the liquid immersion photomask inspection apparatus, where the object
lens is placed below the mask, the entire pattern area of the mask can be
inspected.

[0161]Another example of the liquid immersion object lens used in the
photomask inspection apparatus 100 of FIG. 16 will be described with
reference to FIGS. 21A, 21B and 22. FIGS. 21A and 22B show an object lens
112C different in structure from the object lens 112A of FIGS. 17A and
17B. FIG. 21A is a view of the object lens 112C as seen from above (from
the last lens 115 side), and FIG. 21B is a sectional view thereof. FIG.
22 shows the relative position of the object lens 112C to the mask 108.
Since the components of the photomask inspection apparatus other than the
object lens 112C are the same as in FIG. 16, description thereof will be
omitted. Only the structure related to the supply and retrieval of pure
water of the object lens 112C will be described. In FIGS. 21A, 21B, the
same reference numerals indicate the same components as in the object
lens 112A of FIGS. 17A, 17B with description thereof being omitted.

[0162]The last lens 115 is provided in the center of the top of the object
lens 112C as shown in FIGS. 21A, 21B. The pure water supply port 116 is
provided upstream in the scan direction of the mask near the last lens
115. The pure water supply port 116 is in a shape laterally extending and
round along the outline of the last lens 115 so as to be locatable
adjacent to the circular last lens 115. In the example of FIG. 21A, the
mask is scanned downwards relative to the object lens 112C in the plane
of the page. That is, the object lens 112C moves upwards relative to the
mask 108 as indicated by the broad arrow. As a result, pure water
discharged from the pure water supply port 116 flows over the entire last
lens 115.

[0163]Pure water in contact with the last lens 115 moves away from the
last lens 115 as the mask 108 moves and is sucked into a first pure water
sucking port 117. A small amount of pure water that could not be sucked
into the sucking port 117 is sucked into a second pure water sucking port
118, which is shaped like a long arc surrounding the first pure water
sucking port 117 and the last lens 115. The pure water can be all
retrieved during the scan of the mask with the help of the second pure
water sucking port 118.

[0164]The liquid immersion object lens 112C of the present example differs
from the liquid immersion object lens 112A of FIG. 17A in that it has
only one pure water supply port 116. In this example, when the scan
direction of the mask 108 changes, the liquid immersion object lens 112C
rotates so that the pure water supply port 116 is always located upstream
in the movement direction of the liquid immersion object lens 112C
relative to the mask. That is, as shown in FIG. 22, the orientation of
the object lens 112C changes from 112Ca to 112Cb to 112Cc.

[0165]As described above, in the present example, the liquid immersion
object lens 112C is rotatably attached so as to be able to change its
orientation according to the scan direction of the mask. By this means,
only one pure water supply port 116 is needed, and thus the structure of
the pure water supply and suction part of the liquid immersion object
lens 112C is simplified.

[0166]Further, according to the present example, its orientation can be
set to be in not only a direction (a vertical direction in FIG. 22)
parallel to the scan direction of the mask as indicated by the 112Ca and
112Cc in FIG. 22 but also in a lateral or oblique direction. That is,
during the lateral or oblique movement of the mask as well as the
vertical scan movement, the liquid immersion object lens 112C can operate
in liquid immersion.

[0167]As shown in FIG. 21A, a dry air outlet 119 is provided in the object
lens 112C. By this means, pure water discharged from the pure water
supply port 116 can be prevented from flowing to a great distance into
the other side of the entire last lens 115. That is, while pure water is
being discharged from the pure water supply port 116, dry air ejected
from the dry air outlet 119 can push back the pure water. In particular,
because the way that the pure water spreads can be controlled by the
ejection of dry air, the pure water discharged from the pure water supply
port 116 can be prevented from flowing to a great distance into the other
side of the entire last lens 115 even when the mask is stationary, not
being scanned. Therefore, also while the mask 108 is stopped and a review
function to observe a particular portion is being used, the liquid
immersion object lens 112C can operate in liquid immersion.

Embodiment 8

[0168]A photomask inspection apparatus according to embodiment 8 of the
present invention will be described with reference to FIGS. 23A and 23B.
FIGS. 23A and 23B show the configuration of a mask fixing mechanism of a
photomask inspection apparatus according to the present embodiment. FIG.
23A is a view of the mask fixing mechanism as seen from above, and FIG.
23B is a side view thereof.

[0169]As described previously, the inventors invented a liquid immersion
photomask inspection apparatus configured to observe the pattern surface
from the back surface side of the mask substrate so that the liquid
immersion technique can be applied. However, there is the following other
problem with the liquid immersion photomask inspection apparatus. Where
as shown in FIG. 2 the object lens 212 is placed above the mask 208, when
an edge of the pattern surface 208c of the mask 208 is observed, if
liquid immersion water goes outside the mask substrate 208a, it may flow
around to the opposite side to the liquid immersion surface of the mask
substrate 208a to wet the pellicle 208b.

[0170]A mask fixing mechanism 240 effective in achieving the configuration
of the photomask inspection apparatus of the present invention where the
liquid immersion object lens is placed above the mask will be described
with reference to FIGS. 23A and 23B. The mask fixing mechanism 240
comprises an L-shaped fixing plate 241, fixing plates 242a, 242b, pads
243a to 243g, and arms 244a, 244b.

[0171]The L-shaped fixing plate 241 and fixing plates 242a, 242b are made
of fluorine resin because it is hydrophobic. The L-shaped fixing plate
241 is provided at contact points with the mask 208 with the pads 243a,
243b, 243c. The fixing plate 242a is provided at contact points with the
mask 208 with the pads 243d, 243e. The fixing plate 242b is provided at
contact points with the mask 208 with the pads 243f, 243g. The arms 244a,
244b are attached to the middle of the fixing plates 242a, 242b
respectively and are movable and somewhat rotatable. The thickness of
pads 243a to 243g is as very thin as about 0.1 mm. That is, the distance
between the mask 208, and the L-shaped fixing plate 241 and the fixing
plates 242a, 242b is about 0.1 mm.

[0172]When the mask 208 is set in the photomask inspection apparatus of
the present invention to inspect, the mask 208 is pushed in two
orthogonal directions to be positioned relative to the L-shaped fixing
plate 241. To be specific, in FIG. 23A, the mask 208 is pushed against
the two thin pads 243b, 243c attached to the L-shaped fixing plate 241
downwards by the fixing plate 242a with the two thin pads 243d, 243e
attached to the fixing plate 242a in contact with the mask 208.
Meanwhile, in FIG. 23A, the mask 208 is pushed against the thin pad 243a
attached to the L-shaped fixing plate 241 in the right-to-left direction
by the fixing plate 242b with the two thin pads 243f, 243g attached to
the fixing plate 242b in contact with the mask 208.

[0173]As shown in FIG. 23B, the tops of the L-shaped fixing plate 241 and
the fixing plates 242a, 242b are substantially the same in height as the
top of the mask substrate 208a of the mask 208. Hence, when going outside
the mask 208, the liquid immersion object lens 212 does not interfere
with the L-shaped fixing plate 241 or the fixing plate 242a, 242b.

[0174]As described above, the L-shaped fixing plate 241 and fixing plates
242a, 242b are made of hydrophobic fluorine resin, and the thickness of
pads 243a to 243g is as very thin as about 0.1 mm. Hence, even if pure
water spilt over the mask 208 spreads to the L-shaped fixing plate 241 or
the fixing plate 242a, 242b, it does not drop through those gaps. Thus,
when the liquid immersion object lens 212 is located over an edge of the
pattern area of the mask 208, the pure water 214 from the liquid
immersion object lens 212 does not drop off the mask 208. Therefore,
liquid immersion water from the liquid immersion object lens 212 can be
prevented from flowing around to the pellicle 208b side of the mask 208.

[0175]FIG. 24 shows an example of another mask fixing mechanism 250 that
is a further simplified version of the mask fixing mechanism 240 of FIG.
23A, 24B. As shown in FIG. 24, in the mask fixing mechanism 250, pure
water receiving plates 251a, 251b are attached to short sides of the
rectangular mask 208. That is, the pure water receiving plates 251a, 251b
are located adjacent to the mask 208 to receive pure water going outside
the mask 208. An arm is attached to the middle of the pure water
receiving plate 251a, and the mask 208 is pushed against the pure water
receiving plate 251b downwards by the pure water receiving plate 251a.
The mask 208 is fixed and supported by claws 252a to 252d of an arm at
four corners.

[0176]Where the liquid immersion object lens 212 is placed above the mask
208 as in the present example, not only pure water but also the mask 208
itself may be lifted by the suction by pure water sucking ports provided
in the underside of the liquid immersion object lens 212. Accordingly, in
the present example, as shown in FIG. 23B, the mask 208 is pulled down by
vacuum sucking tubes 245a, 245b (actually four tubes at four corners of
the mask 208), and thereby the mask 208 is not lifted.

[0177]Since the object lens 212 is placed above the mask 208 as shown in
FIG. 23B, the steps 140 as shown in FIGS. 17B and 21B need not be
provided. As a result, multiple pure water supply ports and pure water
sucking ports can be provided in the lower surface of the liquid
immersion object lens in which surface the last lens is provided. Another
example of the liquid immersion object lens will be described with
reference to FIGS. 25A and 25B. FIGS. 25A and 25B shows the configuration
of an object lens 220 used in the present embodiment.

[0178]As shown in FIG. 25A, an last lens 221 is provided in the center of
the lower surface of the object lens 220. Four pure water supply ports
222a to 222d are provided around the last lens 221. Four first pure water
sucking ports 223a to 223d corresponding to the pure water supply ports
222a to 222d are provided outward of the pure water supply ports 222a to
222d. Because the pure water supply ports 222a to 222d and first pure
water sucking ports 223a to 223d are provided at four places
respectively, four scan directions of the mask 208 can be covered. That
is, in the movement in each direction, pure water is supplied from one of
the pure water supply ports 222a to 222d which is located upstream in the
movement and is sucked into one of the pure water sucking ports 223a to
223d which is located downstream in the movement. A ring-shaped second
sucking port 224 is provided surrounding the pure water supply ports 222a
to 222d and the first pure water sucking ports 223a to 223d.

[0179]As described above, in the photomask inspection apparatus of the
present invention, where the liquid immersion object lens is placed above
the mask, multiple supply ports and sucking ports for liquid immersion
water can be provided in the liquid immersion object lens. Hence, only
the ports corresponding to the movement direction of the mask are
operated without rotating the liquid immersion object lens.

[0180]However, when the mask is stopped and a particular portion is
observed, pure water is preferably supplied from all the four pure water
supply ports 222a to 222d a little each. By this means, pure water easily
comes into contact with the entire surface of the last lens 221.

[0181]This will be described using FIGS. 26A-26C. FIGS. 26A-26C illustrate
pure water supply operation at the review with the liquid immersion
object lens 220. When a mask is scanned from top to bottom in the figure
as shown in FIG. 26A, pure water is discharged from only the pure water
supply port 222a of the liquid immersion object lens 220. Thereby, the
shaded area in the figure and thus the entire surface of the last lens
221 are covered by pure water.

[0182]Meanwhile, in the review or the like at which time the mask is
stopped, if pure water is discharged from only the pure water supply port
222a, only part of the last lens 221 may be covered as shown in FIG. 26B.
In contrast, as shown in FIG. 26c, by all the four pure water supply
ports 222a to 222d discharging pure water, the entire surface of the last
lens 221 is covered by pure water when the mask 208 is stopped.

[0183]Although liquid immersion water used in the liquid immersion
photomask inspection apparatus of the present invention is preferably
pure water, ozone water may be used. Because ozone water is high in the
capability of dissolving organic impurities, organic impurities sticking
to the mask substrate can be dissolved during mask inspection. Thus,
performing mask inspection also has the effect that the mask substrate is
cleaned, thus preventing haze or the like that would otherwise occur on
the mask substrate from occurring.

[0184]Concerning the liquid for the liquid immersion which is to be filled
between the last lens and the rear surface of a mask substrate, diluted
alcohol can be used instead of pure water. Alcohol, such as ethanol and
methanol, has a refractive index of 1.61-1.62 at the wavelength of 193
nm. Pure water has a refractive index of 1.436 at the wavelength of 193
nm. Therefore refractive index of diluted alcohol which is mixture of
alcohol and water can be equal to that of the material of the mask
substrate. In this embodiment, quartz of about 1.56 in refractive index
is used as the mask substrate, so refractive index of diluted alcohol can
be about 1.56.

[0185]Using such diluted alcohol, optical length between the pattern
surface of the mask substrate and the objective lens can be constant
independently of the thickness variation of the mask substrate. Therefore
some other solvent can also be used as far as its refractive index is
equal to the refractive index of quartz.

[0186]Also in the case of using such refractive index adjusted liquid, an
auto-focusing (AF) mechanism of the following technique can be used. A
laser beam is supplied from the opposite side of the objective lens and
is radiated on the surface of pattern side of the mask.

[0187]As described above, a practical liquid immersion photomask
inspection apparatus is achieved by the photomask inspection apparatus
according to the present invention, and hence 32 to 22 nm generations of
lithography can be covered. Moreover, in the photomask inspection
apparatus of the present invention, a high NA of 1 is achieved by the
liquid immersion scheme, and even when optical simulation for exposure
through a mask mounted in an exposure apparatus is performed with use of
the review function to observe a particular area on the mask with the
liquid immersion object lens of the present invention without scanning,
the area can be observed with the high NA of 1, and hence optical
simulation for the exposure by the liquid immersion exposure apparatus
can be performed.

[0188]From the invention thus described, it will be obvious that the
embodiments of the invention may be varied in many ways. Such variations
are not to be regarded as a departure from the spirit and scope of the
invention, and all such modifications as would be obvious to one skilled
in the art are intended for inclusion within the scope of the following
claims.